From a surgical point of view many tumors in the brain, e.g. in the pituitary gland, or in organs such as a lung or the liver have until now often been considered as inoperable because they are difficult to access. For a number of years modern beam technology has been used here. The magic word is: Cyberknife. See, for example, J. Frie; Medicine for Managers; Vernissage-Verlag, Heidelberg; Munich 2007 edition.
This is understood to mean a robot arm, similar to the ones used in automotive production, only that the gripper hand is replaced by a special medical irradiation unit. The robot arm can be moved about 6 axes and has specified position accuracy of 0.2 mm. The movements of the patient during irradiation, e.g. due to respiration, are detected by cameras and compensated. For this purpose 3-4 markers that transmit red light signals are arranged over the patient's chest and the cameras measure their position. In addition, by means of two X-ray devices mounted on the ceiling the so-called adiabatic movements such as relaxation of the spinal column, cramping and pains are detected and corrected by the robot's positioning system. By means of the irradiation unit photon beams generated by a linear accelerator are then blasted onto the tumor in the calculated irradiation directions. The duration and strength of irradiation depends on the type of tumor and its size. The beams thereby strike the tumor sitting in the focal point of the beams from e.g. 100 (of 1200 possible) different irradiation directions. By means of the stereotactic irradiation the beam scalpel only applies its deadly effect to the point of the tumor. The ionizing, high-energy photon radiation causes damage to the genetic material (DNA) in the tumor cells, which ultimately leads to the death of the cell. The irradiated healthy tissue in the path of the beams outside of the intersection point is not subjected to lasting damage by the one-off and therefore lower dosed radiation. The advantages of this treatment method are manifold. Surgical intervention and anesthesia are not required. It is an outpatient treatment and the patient can return to his normal daily life immediately after the treatment.
For the radio frequency (RF) acceleration field of the electrons a frequency of 2.998 GHz has become the standard. However, considerably higher frequencies are desirable in order to be able to reduce both the weight and the size of the accelerator unit. Therefore, the electron linear accelerator in the Cyberknife is operated at a frequency of 9.3 GHz. This is an essential requirement for the mobility of the unit. However, the disadvantage of higher frequencies is the reduced power generation of the RF sources. Thus the electron linear accelerator in the Cyberknife provides maximum acceleration energy of 6 MeV. Moreover, by means of the freedom of movement of the irradiation unit in the Cyberknife only magnetrons can be used to generate the RF acceleration field. However, these have a lower output power than klystrons which can only be used statically by the system. The field of application for the latter is preferably large, static irradiation units which achieve acceleration energies of 6 to 23 MeV. Therefore it depends on the type of tumor and the physical condition of the patient how irradiation is to be implemented and which irradiation equipment is used. The electron beam must strike the photon target accurately at the output of the acceleration tube so that the photon radiation most frequently used for irradiation is produced by the electrons accelerated to the speed of light. Deviations in the micrometer range already lead to particle loss or asymmetries in the applied dose profile. In this case it can no longer be guaranteed that the patient will be irradiated with the predetermined radiation dose and that the desired therapy success will be achieved. The deviation of the electron beam from the ideal path is measured by so-called “beam position monitors”. Magnets then correct the detected deviation or the irradiation is blocked like at the Cyberknife if a specific deviation is exceeded. Within the framework of this invention new concepts for the design of the beam position monitor are being investigated, realized and placed in operation. Particular value is placed on the choice of technologies used to be able to produce new systems suitable for the industry.
FIG. 1 shows in principle the structure of an electron linear accelerator 100. Its essential components are: electron radiation source 110, high frequency source 120, acceleration tube 130, photon target 140. A classic electron radiation source, e.g. the electron gun, has a combination of thermal electron cathode and the optical beam elements, which enable temporal and spatial bundling of the primary electrons. In the first two cells of the accelerator, in the so-called “buncher cells”, the electrons are bundled and then accelerated by an electromagnetic field 150 with a longitudinal field portion to almost the speed of light. A circular waveguide is preferably used as acceleration tube and is fed with the E01 basic mode. Either a magnetron or a klystron is used as RF source. After leaving the linear accelerator (LINAC) the electrons 160 strike a heavy metal target, generally tungsten, with an energy of 6 to 23 MeV, and the photon radiation most frequently used for the irradiation of tumors is produced. A detailed derivation of the following fundamental physical aspects of electron acceleration can be found in Krieger, Hanno; Radiation Sources for Technology and Medicine; Wiesbaden, Teubner; 2005, and Wille, Klaus; The Physics of Particle Accelerators and Synchrotron Radiation Sources; Stuttgart, Teubner; 1996.
The electromagnetic wave that accelerates the electron beam is generally generated and amplified by a magnetron or klystron with a transmitting frequency of 2.998 GHz. The magnetron or klystron couples into a rectangular wave-guide in the H10 mode. The coupling from the rectangular wave-guide into the E01 mode of the circular waveguide of the acceleration tube then takes place for matching reasons through a slot because the field configurations are the same at the coupling-in point. The extremely high RF output power that is required to accelerate the electrons to almost the speed of light can only be made available in the pulse operation of the magnetron or klystron for thermal reasons. Therefore, electron bundles are fed into the acceleration tube in proper phase relation by the electron gun. The bundles have a running time of 5 μs, and within this running time single pulses with a pulse duration of 30 ps and a repetition rate of 333 ps. The repetition rate corresponds to a frequency of 3 GHz. After the pulse there is no signal for 5 to 20 ms. FIG. 2 shows the development of the signals over time.
There are 2 types of electron linear accelerators: the travelling-wave and the standing-wave accelerator. According to the travelling wave principle the electrons are accelerated at the crest of the radio-frequency wave when coupled in the proper phase relation. The speed of the electrons that are located just in front of the wave maximum is therefore continuously increased over the whole length of the acceleration tube. The electrons run with the wave. In standing-wave accelerator the length of the acceleration tube is designed so that a standing wave can form in the tube (at the end of the acceleration tube) by reflection of the wave at the end of the acceleration tube. Since the wave troughs would cause negative acceleration of the electrons, over the temporal course of the acceleration the wave has undergone a phase shift of e.g. 180 degrees as soon as the electrons to be accelerated pass into the respective next resonance chamber. It is thus guaranteed that the electrons are always accelerated in the beam direction.
Referring to FIG. 3, a standing-wave accelerator 300 includes a drift tube 310, resonance chambers 320, and coupling cavities 330. According to the standing wave principle, the relocation to the side of the electromagnetic wave in the zero passages into so-called coupling cavities enables considerable shortening of the acceleration tube. While the electromagnetic wave couples into the next resonance chamber through the coupling cavities, the electron beam 340 gets there through a so-called drift section tube. The drift section tube has dimensions such that the 3 GHz E01 mode is not propagable, i.e. it lies below the limit frequency. Therefore, the drift section tube of the electron beam between the resonators can be designed according to the requirements of the beam optics and is an ideal place for measuring the position of the electron beam using coupling probes and then for correcting the deviation by means of magnets along the accelerator tube.